An ion trap is used in a mass spectrometer in order to capture and confine ions by the effect of a radio-frequency electric field, select an ion having a specific mass-to-charge ratio (m/z) well as fragment the ion selected in such a manner. A typical ion trap is a three-dimensional quadrupole ion trap formed by a single ring electrode having an inner surface in the form of a hyperboloid of one sheet as well as a pair of end-cap electrodes having an inner surface in the form of a hyperboloid of two sheets facing each other across the ring electrode. Another commonly known type is a linear ion trap formed by four rod electrodes arranged parallel to each other. In the present description, the “three-dimensional quadrupole type” is used as an example of the ion trap for convenience of explanation.
In a conventional and common type of ion trap, a sinusoidal radio-frequency voltage is normally applied to the ring electrode to create a radio-frequency ion-capturing electric field within the space surrounded by the ring electrode and the end-cap electrodes so as to confine ions by the radio-frequency electric field while oscillating the ions. Meanwhile, in recent years, a type of ion trap which confines ions by applying a rectangular voltage to the ring electrode in place of the sinusoidal radio-frequency voltage has been developed (for example, see Patent Literature 1, Patent Literature 2 or Non-Patent Literature 1). This type of ion trap is called the “digital ion trap (DIT)” since it normally uses a rectangular voltage having the binary voltage levels of “High” and “Low”.
In the conventional analogue-driven type of ion trap, an LC resonator is used to generate the sinusoidal radio-frequency voltage. The mass-to-charge-ratio range of the ions that can be captured is controlled by regulating the amplitude of the sinusoidal radio-frequency voltage. On the other hand, in the case of the digital ion trap, the rectangular high-frequency voltage is generated by the high-speed switching of two direct voltages. The mass-to-charge-ratio range of the ions that can be captured is controlled by changing the frequency of the rectangular voltage while constantly maintaining the amplitude of the voltage. This allows the amplitude of the high voltage applied to the ring electrode to be lower than in the case of the analogue-driven type, so that the circuit for generating the radio-frequency voltage can be created at a lower cost. Another advantage is that the generation of unwanted electric discharge between the electrodes can be avoided.
The rectangular voltage applied to the ring electrode in the previously described digital ion trap is normally switched between ±several hundred volts or ±several kilovolts. Its frequency is varied within a considerable range, from several ten kHz to several MHz. In order to generate such a rectangular voltage, a high-speed semiconductor switching element, such as a power MOSFET, is used in the radio-frequency voltage generation circuit to switch between a positive voltage and a negative voltage (see Patent Literature 2 or Non-Patent literature 1). Such a semiconductor switching element (which is hereinafter simply called the “switching element”) generates a certain amount of heat during the switching operation. Therefore, the temperature of the switching element used in a digital ion trap will be considerably high. This temperature increases with the frequency of the switching operation.
In a mass spectrometer using the previously described type of ion trap, it has generally been the case that a rectangular voltage having a low frequency (e.g. equal to or lower than 20 kHz) which significantly deviates from the normal frequency range used for capturing ions is applied to the ring electrode during a standby state in which no analysis is being undertaken, in order to completely remove unwanted ions remaining within the ion trap. When an analysis is initiated from such a standby state, the frequency of the rectangular voltage applied to the ring electrode is increased, so that the temperature of the switching element becomes higher than in the standby state. Such a change in the temperature causes a change in the on-state resistance or other electric characteristics of the switching element, which in turn causes a slight yet certain change in the amplitude of the rectangular voltage. Therefore, after the frequency of the rectangular voltage is switched from the low frequency to the high frequency for an analysis, the amplitude of the rectangular voltage will gradually change (i.e. drift) with the increasing temperature of the switching element until this temperature is stabilized.
An analysis by a mass spectrometer is normally performed as follows: A process which includes the successive steps of generating ions, introducing the ions into an ion trap, as well as ejecting and detecting the ions by a mass scan is repeatedly performed for one sample. A mass profile is obtained by each mass scan, and the obtained mass profiles are accumulated in a computer to obtain a mass spectrum with a high signal-to-noise ratio (for example, see Patent Literature 3). The timing at which an ion having a certain mass-to-charge ratio is ejected from the ion trap in the mass scan depends on the frequency and amplitude of the rectangular voltage. Therefore, if the amplitude of the rectangular voltage gradually changes due to the temperature change as described earlier, the point in time of the ejection of the ion having the same mass-to-charge ratio gradually shifts with the repetition of the mass scan. Accumulating mass profiles obtained with such a shift will result in a deterioration of the mass resolution of the mass spectrum.
The present inventor has proposed an ion trap device having the function of reducing the drift of the ion-ejection time in the mass scan, as disclosed in Patent Literature 4. In the ion trap device described in that document, after an analysis of one sample has been completed, the reaching temperature of the switching element in the next analysis is predicted, and the switching element is turned on and off at a frequency required for maintaining that temperature during the standby period until the analysis for the next sample is initiated. This operation reduces the amount of change in the temperature of the switching element at the transition from the standby state to the next analysis, and thereby decreases the drift of the ion-ejection time due to the temperature change (it should be noted that the ions remaining within the ion trap are completely removed by lowering the frequency for a short period of time immediately before the execution of the next analysis).    Patent Literature: 1: IP 2007-527002 A    Patent Literature 2: JP 2008-282594 A    Patent Literature 3: WO 2008/129850 A    Patent Literature 4: JP 2011-023167 A    Non-Patent Literature 1: Furuhashi, Takeshita, Ogawa, Iwamoto, Ding, Giles, and Smirnov, “Dejitaru Ion Torappu Shitsuryou Bunseki Souchi No Kaihatsu (Development of Digital Ion Trap Mass Spectrometer)”, Shimadzu Hyouron (Shimadzu Review), Shimadzu Hyouron Henshuubu, Mar. 31, 2006, Vol. 62, Nos. 3·4, pp. 141-151